As stated in the first link I posted...
Toggle: 1957-1960
Fatboy:1958-61
Slim: 1961-68
Super: 1969-86

As far as the quality of later Gillette razors my understanding is that the Super is well regarded. I am certainly no expert. If you do a google search on Gillette date codes you should be able to find some of the info you were asking about. Razor Emporium has some good articles and videos. You might be surprised what a google search and some surfing turn up. Badger and Blade is a good resource for info on DEs and SEs. You could also try searching this forum. There is a lot of good info tucked away in old threads, not to mention the library. Have fun!

Just out of curiosity what quarter is your 1975 from?